1. Field of Invention
This invention relates generally to an apparatus and method for enhancing productivity from a subterranean hydrocarbon bearing formation, and in particular to an apparatus and method for improving the productivity of hydrocarbons produced from a subterranean formation through perforations in a well penetrating the formation.
2. Description of Related Art
In completing a well for producing fluids from a subterranean formation, it is common to install a casing, cement the casing to the well bore face, and then perforate the casing and cement by detonating shaped explosive charges. The perforations thus formed extend through the casing and the cement a short distance into the formation. In some formations, it is desirable to conduct the perforating operations with the pressure in the well overbalanced with respect to the formation pressure. Under overbalanced conditions, the well pressure exceeds the pressure at which the formation will fracture, and hydraulic fracturing occurs in the vicinity of the perforations. The perforations may penetrate several inches into the formation, and the fracture network may extend several feet into the formation. Thus, an enlarged conduit can be created for fluid flow between the formation and the well, and well productivity may be significantly increased by deliberately inducing fractures at the perforations.
When the perforating process is complete, the pressure within the well is allowed to decrease to the desired operating pressure for fluid production or injection. As the pressure decreases, the newly created fractures tend to close under the overburden pressure. One approach to ensuring that fractures and perforations remain open conduits for fluids flowing from the formation into to the well or from the well into the formation is to inject particulate material into the perforations to prop the fractures open. The proppant can be emplaced either simultaneously with formation of the perforations or at a later time. For example, the lower portion of the wellbore can be filled with a sand slurry prior to perforation. The sand is subsequently driven into the perforations and fractures by the pressured fluid in the wellbore during conventional overbalanced perforating operations. In addition to propping the induced fractures, the sand may also scour the surface of the perforations and/or the fractures, thereby enlarging the conduits created for enhanced fluid flow.
Problems encountered with prior art fracturing methods include (1) difficulty in maintaining adequate fluid pressure to enable the proppant to enter the fracture and (2) the need to use relatively large quantities of liquid and proppant. One solution to the first problem is to mount a container, or dump bailer, of sand above a perforating gun in a well with overbalanced conditions. Simultaneously with detonation of the perforating charges, the sand is released into the well by rupturing the bottom of the dump bailer and carried into the perforations by the pressured liquid. Another method emplaces the sand after perforation by applying mechanical or explosive pressure to a combination of particulate matter and liquid in the wellbore adjacent the perforations.
Prior art methods of propping fractures in conjunction with the perforating operation have generally utilized wireline devices or assemblies which are lowered into the well on an electrical cable in communication with instrumentation at the surface. The limited strength of the wireline limits the length of the perforation assembly that can be lowered, thereby also limiting the length of the interval in the well that can be perforated at one time. Decreased operating costs could be achieved if longer intervals could be perforated in one trip into well. Fewer trips into the well would also decrease the risk of accidents due to well blowouts during high pressure operations and less handling of explosives for perforating. Further, it is often desirable to perforate longer intervals in horizontal wells than are commonly encountered in vertical wells.
The use of wireline devices also limits the weight of the perforating string and the pressure which can be applied to the zone being perforated. Because packers cannot be used to isolate zones within the well in conjunction with a wireline, the entire well must be subjected to the pressure required for the perforating/fracturing operation. Thus, the pressure must be limited to a pressure which will not damage the weakest part of the well. For example, the high pressure required to fracture an interval may damage another, previously completed, interval in the well. Further, a relatively large quantity of liquid and gas may be required to pressurize the entire well, increasing the cost of the operation.
Wireline conveyance of perforating devices is also unsatisfactory when perforating high-angle and horizontal wells. It is well known in the art that most downhole tools, including perforating devices, cannot be positioned properly using wirelines in high-angle sections of wells. A more rigid conveyance means, such as tubing, must be utilized.
Thus, it is an object of the present invention to enable improved perforation, fracturing, and propping of longer intervals of a subterranean well and formation in a single operation, using small quantities of fluid and proppant.
It is also an object of the present invention to confine pressurized fluid to an isolated zone in a well during perforation.
It is an additional object of the present invention to confine higher fluid pressure to an isolated zone in a well during perforation.
It is yet another object of the present invention to provide a means of perforating and fracturing intervals in high-angle and horizontal wells, utilizing tubing-conveyed perforating equipment.